volatile unsigned char rxb[256], rxc=0, prxc=0;

unsigned char serial_available(void)
{
	if(prxc!=rxc)
	{
		return(1);
	}
	else
	{
		return(0);
	}
}

unsigned char serial_read(void) //blocking
{
	while(prxc==rxc); //If you always call and test for serial_available before reading this can be removed
	return(rxb[prxc++]);
}

unsigned char serial_read(unsigned char *b) //nonblocking
{
	if(prxc!=rxc)
	{
		*b=rxb[prxc++];
		return(1);
	}
	else
	{
		return(0);
	}
}

ISR(USART_RX_vect)
{
	rxb[rxc]=UDR0;
	rxc+=1;
}